This invention relates generally to conductors having ceramic insulation applied thereto, and a method of applying such insulation using a sol-gel method. In particular, the invention relates to the application of ZrO2, MgOxe2x80x94ZrO2, Y2O3xe2x80x94ZrO2, CeO2xe2x80x94ZrO2, In2O3xe2x80x94ZrO2 and SnO2xe2x80x94ZrO2 coatings on high temperature superconducting (HTS) conductors, e.g., Ag or AgMg sheathed Bi-2212, Bi-2223 and like tapes, and bronze processed (or like) multifilamentary low temperature superconducting (LTS) (e.g., Nb3Zn, Nb3Ge, Nb3Al, V3Ga, V3Si, (NbTi)3Sn, Nb3Sn1xe2x88x92xGax, and like material) conductors. Other LTS conductors also include Cu(or CuSn)xe2x80x94Nb3xe2x80x94Sn wires using a sol-gel technique.
In the manufacture of HTS and LTS magnets, it is often desirable to use composite tapes employing high temperature compatible insulating materials. Specifically, pancake coils are wound from HTS conductors in which a xe2x80x9cwind and reactxe2x80x9d approach is used due to the brittle nature of the materials. As a result, it became necessary to use high temperature compatible insulating materials for turn-to-turn insulation in any magnet built from the high temperature superconductors. In the past, ceramic based papers or tapes were used for this purpose. However, a disadvantage with this type of approach is that such insulation, of course, takes up valuable winding space, and currently available materials have a thickness of about 0.1 mm. or greater.
The wind and react method of the prior art involved winding the precursor to a superconducting material around a mandrel in order to form a coil, and then processing the coil with high temperatures in an oxidizing environment. The processing method results in the conversion of the precursor material to a desired superconducting material, and in the healing of micro-cracks formed in the precursor during the winding process, thus optimizing the electrical properties of the coil. The superconducting coils, like most coils, are formed by winding an insulated conducting material around a mandrel defining the shape of the coil. When the temperature of the coil is sufficiently low that the conductor can exist in a superconducting state, the current performance of the conductor is increased and large magnetic fields can be generated by the coil.
As is well known, certain ceramic materials exhibit superconducting behavior at low temperatures, such as the compound Bi2Sr2CanCun+1O2n+4 where xe2x80x9cnxe2x80x9d can be either 0, 1, and 2. One material, Bi2Sr2Ca2Cu3O10 (BSCCO(2223)), has performed particularly well in device applications because superconductivity and corresponding high current densities are achieved at relatively high temperatures, Tc=115xc2x0 K. Other oxide superconductors include general Cuxe2x80x94O-based ceramic superconductors, such as members of the rare-earth-copper-oxide family (i.e., YBCO), thallium-barium-calcium-copper-oxide family (i.e., TBCCO), the mercury-barium-calcium-copper-oxide barium-calcium-copper-oxide family (i.e., HgBCCO), and BSCCO compounds containing lead (i.e., Bi, Pb)2Sr2Ca2Cu3O10).
Insulating materials surrounding the conductors are used to prevent electrical short circuits within the winding of a coil. From a design point of view, the insulation layer must be able to withstand large electric fields (as high as 4xc3x97105 V/cm in some cases) without suffering dielectric breakdown, a phenomenon that leads to electrical cross-talk between neighboring conductors. At the same time, in the past, it was desired to make insulation layers as thin as possible (typically 50-150 xcexcm) so that the amount of superconducting material in the coil can be maximized.
By using existing conducting and insulating materials, the maximum magnetic field generated by a superconducting coil is ultimately determined by the winding density (defined as the percentage of the volume of the coil occupied by the conductor) and the coil geometry. However, the large tensional forces necessary for high winding densities can leave conductors in highly stressed and/or strained states. The bend strain of a conductor, equal to half the thickness of the conductor divided by the radius of the bend, is often used to quantify the amount of strain imposed on the conductor through coil formation.
Thus, instead of the xe2x80x9cwind and reactxe2x80x9d process previously discussed, one prior method used to fabricate coils with multi and mono-filament composite conductors is the xe2x80x9creact and windxe2x80x9d process. This method first involves the formation of a insulated composite conductor which is then wound into a coil. A precursor to a composite conductor is fabricated and placed in a linear geometry, or wrapped loosely around a coil and placed in a furnace for processing. The precursor can therefore be surrounded by an oxidizing environment during processing, which is necessary for a conversion to the desired superconducting state. In the xe2x80x9creact and windxe2x80x9d processing method, insulation can be applied after the composite conductor is processed, and materials issues such as oxygen permeability and thermal decomposition of the insulating layer do not need to be addressed.
In the xe2x80x9creact and windxe2x80x9d process, the coil formation step can, however, result in straining composite conductors in excess of the critical strain value of the conducting filaments. Strain introduced to the conducting portion of the wire during the deformation process can result in micro-crack formation in the ceramic grains, severely degrading the electrical properties of the composite conductor.
Alternatively, in the xe2x80x9cwind and reactxe2x80x9d process previously discussed, the eventual conducting material is typically considered to be a xe2x80x9cprecursorxe2x80x9d until after the final heat treating and oxidation step. Unlike the xe2x80x9creact and windxe2x80x9d process, the xe2x80x9cwind and reactxe2x80x9d method as applied to high temperature superconductors requires that the precursor be insulated before coil formation, and entails winding the coil immediately prior to a final heat treating and oxidation step in the fabrication process. This final step results in the repair of micro-cracks incurred during winding, and is used to optimize the superconducting properties of the conductor. However, these results are significantly more difficult to achieve for a coil geometry than for the individual wires which are heat treated and oxidized in the xe2x80x9creact and windxe2x80x9d process.
Due to the mechanical properties of the conducting material, superconducting coils fabricated using the xe2x80x9cwind and reactxe2x80x9d approach with composite conductors have limitations related to winding density and current-carrying capability. Although the xe2x80x9cwind and reactxe2x80x9d process may repair strain-induced damage to the superconducting material incurred during winding, the coils produced are not mechanically robust, and thermal strain resulting from cool down cycles can degrade the coil performance over time. Moreover, currently available insulation takes up a lot of winding space limiting the number of turns achievable, and further limiting the teslas at the highest field achievable in the bore of a magnet.
Currently, high temperature superconductors are produced using a powder in tube process which results in multifilamentary tapes or round wires of Bi-2212, Ag (or AgX where X=Mg, Zr, Al, Au, Y, etc.) or Bi-2223, Ag (or AgX). In the parent application to this application a sol-gel process is described to provide insulation to such tapes. In that process since zirconium oxide is already formed as an insulation layer, rolling is required to achieve a physical bond between the insulation and the coating. This is a complex procedure and sometimes does not result in a satisfactory bond.
Similarly, in the insulating of low temperature superconductors, wires are braided with a glass insulation applied. After heat treatment the glass insulation typically becomes very brittle, and often easily breaks off. This is an undesirable result.
The current invention avoids the problems of the prior art by providing a process where a chemical bond is established between the conductor and the insulation for both HTS and LTS conductors. Thus, a good bond is established and the problems of the prior art avoided.
In accordance with one aspect of the invention, there is provided a method of coating a conductor with an insulating composition. The method includes the steps of preparing a sol-gel coating solution with one of a Zr composition or a Zr composition with one of Mg, Y, Ce, In, and Sn composition based precursor materials in combination with a solvent and a catalyst. A conductor, which can be a high temperature superconductor such as a tape, or a low temperature superconductor such as a wire, as described herein, is dipped into the solution to coat the conductor with the solution. The solution is then dried at a temperature and for a time sufficient to evaporate the solvent. The coated conductor is then heated in an oxygen containing environment, such as air, at a temperature and for a time sufficient to result in a ceramic insulation coating. More preferably, the drying is conducted at a temperature of about 300xc2x0 C. and the heating is conducted at a temperature of about 500xc2x0 C. to about 650xc2x0 C. Depending on the precursor, this results in an insulating coating which is either ZrO2, MgOxe2x80x94ZrO2, Y2O3xe2x80x94ZrO2, CeO2xe2x80x94ZrO2, In2O3, ZrO2 or SnO2. Similarly, Bi-2223 and like tapes can be insulated in accordance with the invention. Likewise, bronze processed (or like) multifilamentary low temperature superconducting (e.g., Nb3Zn, Nb3Ge, Nb3Al, V3Ga, V3Si, (NbTi)3Sn, Nb3Sn1xe2x88x92xGax, and like materials) can be the conductors with which invention is practiced.
In a more specific aspect, the conductor is Ag/Bi-2212 tape, AgMg/Bi-2212 tape, Cu/Nb3Sn wire or Cuxe2x80x94Sn/Nb3Sn wire.
The invention also relates to a conductor having a ceramic insulative coating, made in accordance with the method described above. The conductors and coatings are as also previously described.